(i) Field of the Invention
The present invention relates to an oxygen absorbing resin, a deoxidizing multi-layer structure using this resin, and a packaging containers This oxygen absorbing resin can be obtained by dispersing a deoxidizing composition in a resin having flexibility and a high oxygen permeability, and in addition, the oxygen absorbing resin has an oxygen absorbing velocity equal to that of a deoxidizing agent received in a small bag (hereinafter referred to as "the deoxidizing agent in the small bag" on occasion) and a higher oxygen absorbing ability as compared with a conventional oxygen absorbing resin. Furthermore, the deoxidizing multi-layer structure using this oxygen absorbing resin is excellent in the oxygen absorption ability and is also moldable. Moreover, the packaging container of the present invention can be utilized to receive drinks, foods, medicines and the like which are easily affected by oxygen, and this packaging container can shut off external oxygen and can remove oxygen present within the container itself in a short period of time without previously purging the container with an inert gas or the like, whereby the spoilage of the packed material can be prevented and hence its long-term preservation is possible.
(ii) Description of the Related Art
Heretofore, as a technique of removing oxygen from packages receiving packed materials such as foods and medicines to prevent the oxidation deterioration, the color change and the like of the packed materials, the spoilage of the packed materials by mold and aerobic bacteria, and the damage of the packed materials by worms, a way of putting a deoxidizing agent together with the material to be packed in each gas barrier bag and then sealing the bag has widely been carried out. As this way, there has usually been used the deoxidizing agent in which a powdery or a particulate deoxidizing composition is filled in a small bag having ventilation properties. However, when this deoxidizing agent in the small bag is mixed with the material to be preserved, the deoxidizing agent might be cooked together with the material to be preserved and then eaten, or the mixed deoxidizing agent might give an alien feeling or a resistant feeling to a user. In addition, the small bag might be mistakenly handled to tear the same, and at this time, the scattered deoxidizing composition might contaminate the material to be preserved. Accordingly, as a deoxidizing package technique not having the above-mentioned problems, it has been contrived in recent years to impart a deoxidizing ability to the packages themselves.
As such examples, for instance, as disclosed in Japanese Patent Publication No. 1824/1987 and Japanese Patent Application Laid-open Nos. 146651/1982, 45152/1992, 90848/1992 and 309323/1995, there are films and sheets of a multi-layer structure having a layer in which the deoxidizing composition is dispersed in a resin. In addition, Japanese Patent Publication No. 60826/1992 has disclosed a packaging container in which the commercially available deoxidizing composition is contained in a gas barrier thermoplastic resin. Furthermore, in Japanese Patent Publication No. 62858/1992, there has been disclosed a container containing a material which can express an oxygen capturing function only at a high temperature and under a high water vapor pressure in a retort treatment. Japanese Patent Publication Nos. 60826/1992 and 62858/1992 have disclosed a technique in which the deterioration of the oxygen barrier properties of a gas barrier resin in the retort treatment is restrained to prevent oxygen from getting into the container.
However, in the packaging container to which an oxygen absorbing performance is imparted, the deoxidizing composition is dispersed in the resin, and therefore the characteristics of the resin have a large influence on the oxygen absorbing velocity and the maximum oxygen absorption of the deoxidizing composition. In the conventional packaging container to which the oxygen absorbing performance is imparted, oxygen penetrated through the resin is absorbed by the deoxidizing composition. Therefore, the oxygen absorbing performance of this packaging container is much lower as compared with the conventional technique using the deoxidizing agent in the small bag, and so there has been a problem that oxygen present in the packaging container cannot be efficiently removed in a short period of time. For the purpose of overcoming such a drawback, for example, there has been used, together with the above manner, a technique in which air of the space in the package is replaced with an inert gas prior to packing the packaging container having the oxygen absorbing performance with the material to be preserved so as to previously decrease an oxygen concentration. For the replacement with the inert gas, however, specific facilities are required, which leads to the increase of a cost. Thus, the above-mentioned problem has not always fundamentally been solved.
In view of the above-mentioned conventional techniques, the present inventors have intensively researched with the intention of improving the deoxidizing performance of the oxygen absorbing resin in which the deoxidizing composition is dispersed. As a result, it has been found that an oxygen absorbing resin obtained by dispersing the deoxidizing composition in a thermoplastic resin mixture containing a thermoplastic elastomer and an ethylene propylene copolymer and having flexibility and a high oxygen permeability constant has a much higher oxygen absorbing velocity as compared with the conventional oxygen absorbing resin and a maximum oxygen absorption several times as much as that of the conventional oxygen absorbing resin. In addition, it has also been found that a deoxidizing multi-layer structure obtained by laminating a layer comprising a thermoplastic resin having a high oxygen permeability, a layer comprising the above-mentioned oxygen absorbing resin and a gas barrier layer, and a packaging container obtained by molding this deoxidizing multi-layer structure can exert the practical oxygen absorbing performance equal to that of the deoxidizing agent in the small bag. The present invention has been completed on the basis of the found knowledge.